1. Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly, to a liquid crystal display device which has a liquid crystal display panel, a light guide disposed on the back surface of the liquid crystal display panel, and linear light sources respectively disposed along a pair of side edges of the light guide.
2. Description of the Related Art
A liquid crystal display device capable of providing high-resolution and color display for notebook personal computers and computer monitors is provided with a light source which illuminates a liquid crystal display panel from the back surface thereof (that is, a backlight). A generally known type of illumination light source for the liquid crystal display panel is a side edge backlight in which a linear light source is disposed along a side (side edge) of a planar light guide plate or a direct backlight in which a plurality of linear light sources are directly disposed on the back surface of the liquid crystal display panel.
The side edge backlight includes at least one linear light source disposed along at least one side edge of a light guide plate made of a transparent plate such as an acrylic plate, and an optical sheet made of an optical diffusion plate or a prism sheet stacked on a liquid-crystal-panel side of the light guide plate. The assembly of these members is hereinafter referred to as a backlight structure. This backlight structure is housed in a lower case, and after a liquid crystal display panel has been stacked on the backlight structure, a metal upper case having a display window is placed on and joined to the lower case, whereby the backlight structure and the liquid crystal display panel as well as the lower and upper cases are integrated as a liquid crystal display device (that is, a liquid crystal display module). In general, a so-called cold cathode fluorescent lamp is used as the linear light source.
FIG. 8 is a cross-sectional view diagrammatically illustrating the essential structure of a liquid crystal display device using a backlight structure having cold cathode fluorescent lamps respectively disposed along two opposite side edges of a light guide plate. In FIG. 8, reference symbol BL denotes the backlight structure, and cold cathode fluorescent lamps CFL1 and CFL2 are respectively disposed along two opposite sides of a planar light guide plate GLB which is suitably made of an acrylic resin. The cold cathode fluorescent lamps CFL1 and CFL2 are respectively provided with lamp reflection sheets LS1 and LS2, and an optical reflection sheet RF is provided on a second main surface (back surface) of the light guide plate GLB.
An optical diffusion sheet SPS and a prism sheet PRS are stacked on a first main surface (top surface) of this light guide plate GLB, and a liquid crystal display panel PNL is stacked on the prism sheet PRS with one main surface of the liquid crystal display panel PNL opposed to the prism sheet PRS. The liquid crystal display panel PNL includes a liquid crystal layer sealed between a pair of substrates SUB1 and SUB2, and polarizers POL1 and POL2 are respectively provided on the one main surface (a surface opposed to the light guide plate GLB) and another main surface (an image-display-side surface) of the liquid crystal display panel PNL.
FIG. 9 is a developed perspective view illustrating the entire construction of the liquid crystal display device provided with the side edge backlight. This liquid crystal display device is formed by housing into a lower case MCA the liquid crystal display panel PNL, the light guide plate GLB, the cold cathode fluorescent lamps CFL1 and CFL2 and an optical sheet made of the optical diffusion sheet SPS and the prism sheet PRS, placing an upper case SHD which is a frame-shaped member onto the lower case MCA, and securing, by caulking, pawls NL provided at the periphery of the upper case SHD to engagement portions NA of the lower case MCA to integrate the upper case SHD and the lower case MCA. This engagement is effected by folding the pawls NL inward of the engagement portions NA.
In this side edge type of backlight structure, the thickness and the weight of the light guide plate occupy most of the thickness and the weight of the backlight structure. In recent years, as liquid crystal display modules are reduced in thickness and power consumption, light guide plates are being reduced in thickness. If the thickness of a light guide plate is reduced, the amount of light obtained from one linear light source becomes insufficient, so that one or more linear light sources need to be used on two opposite sides of the light guide plate.